Abscisic acid (ABA) is a plant hormone that regulates signal transduction associated with abiotic stress responses (Cutler et al., 2010). The ABA signaling pathway has been exploited to improve plant stress response and associated yield traits via numerous approaches (Yang et al., 2010). The direct application of ABA to plants improves their water use efficiency (Raedmacher et al., 1987); for this reason, the discovery of ABA agonists (Park et al., 2009; Melcher et al., 2010) has received increasing attention, as such molecules may be beneficial for improving crop yield (Notman et al., 2009). A complementary approach to activating the ABA pathway involves increasing a plant's sensitivity to ABA via genetic methods. For example, conditional antisense of farnesyl transferase beta subunit gene, which increases a plant's ABA sensitivity, improves yield under moderate drought in both canola and Arabidopsis (Wang et al., 2005). Thus, the manipulation of ABA signaling to improve traits contributing to yield is now well established.
It has recently been discovered that ABA elicits many of its cellular responses by binding to a soluble family of receptors called PYR/PYL proteins. PYR/PYL proteins belong to a large family of ligand-binding proteins named the START superfamily (Iyer et al., 2001); Ponting et al., 1999). These proteins contain a conserved three-dimensional architecture consisting of seven anti-parallel beta sheets, which surround a central alpha helix to form a “helix-grip” motif; together, these structural elements form a ligand-binding pocket for binding ABA or other agonists.
Structural and functional studies have uncovered a series of conformational changes and critical contacts between PYR/PYL receptors and type II C protein phosphatases (PP2Cs) that are necessary for ABA-mediated PP2C inhibition by receptors. For example, when ABA or another agonist binds within the ligand-binding pockets of PYR/PYL proteins, it stabilizes a conformational change that allows the receptors to bind and inhibit a family of PP2Cs that normally repress ABA signaling (Weiner et al., 2010). In particular, ABA binding leads to a large rearrangement in a flexible “gate” loop that flanks the ligand-binding pocket. Upon ABA binding, the gate loop adopts a closed conformation that is stabilized by several direct contacts between the loop and ABA. This agonist-bound, closed form of the gate allows PYR/PYL proteins to dock into, and inhibit, the active site of PP2Cs. The resulting inhibition in turn allows activation of downstream kinases in the SnRK2 class, which are responsible for the regulation of the activity of transcription factors, ion channels and other proteins involved in ABA responses (Weiner et al., 2010). Thus, the stabilization of a closed gate conformation of the receptors is critical to their activation and PYR/PYL receptors are molecular switches at the apex of a signaling cascade that regulates diverse ABA responses.
In addition to the important role that gate closure plays in receptor activation, other structural rearrangements are critical as well. For example, PYR1, PYL1, and PYL2 are homodimers in solution, but bind to PP2Cs as monomers. The homodimer interface overlaps with the PP2C binding interface and therefore an intact receptor homodimer cannot bind to and inhibit the PP2C. Thus, dimer formation is antagonistic to ABA signaling and receptor dimer-breaking is a necessary step in receptor activation. Additionally, a recognition module containing a central conserved tryptophan “lock” residue located on the PP2C inserts into a small pore formed in the ABA-bound receptors. Mutation of the tryptophan lock residue abolishes receptor-mediated inactivation of PP2C activity, demonstrating the importance of the lock residue's insertion into the receptor's pore.